Taking antibiotic drugs may raise cancer risk, expert warns

Taking antibiotic drugs may raise the risk of developing colon cancer five to 10 years down the line, according to a new study of more than 40,000 cancer cases in Sweden.

Past studies hinted that antibiotics can cause lasting changes to the gut microbiome — the community of microbes that live in the digestive tract — and that these changes may be linked to a heightened risk of colon cancer.

Now, in the largest epidemiological study to ever explore this link, researchers report that the heightened risk may be specific to cancers in the so-called proximal colon, the part of the colon that connects to the small intestine and starts in the lower-right abdomen.

“It’s very clear, when we look at the data, that it’s very confined to the proximal, or right-sided colon,” senior author Sophia Harlid, a cancer researcher at Umeå University in Sweden, told Live Science. And in fact, the antibiotic-related cancer risk was greatest at the start of the proximal colon, called the “ascending colon,” which extends from the lower- to upper-right abdomen.

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People who took antibiotics for more than six months bore the highest cancer risk, according to the research, published Wednesday (Sept. 1) in the Journal of the National Cancer Institute. Compared with people who’d taken no antibiotics, these individuals had a 17% higher chance of developing cancer in the ascending colon.

That said, even short courses of antibiotics carried an associated cancer risk, albeit a far smaller one than what was seen with the months-long regimens, the team found. This data may provide yet another reason to rein in the over-prescription of antibiotics, besides preventing the emergence of antibiotic-resistant superbugs, Harlid said.

These new findings echo the results of a similar, but smaller, U.K.-based study, published in 2019 in the journal Gut. The Swedish study “came right in line with other data that was emerging … which actually improves confidence that there’s an association,” Dr. Cynthia Sears, senior author of the U.K. study, who was not involved in the newest research, told Live Science.

“Our thinking is that you’re disrupting the balance of the microbiota,” and this may allow infectious bugs like Escherichia coli and Klebsiella pneumoniae to gain prominence where they’d usually be outcompeted by other microbes, Sears said. This in turn may ramp up inflammation in the colon, generating reactive chemicals that could damage DNA and generate tumors. In addition, the inner lining of the intestine may then become more permeable, allowing bacteria to infiltrate the colon walls and join together in slimy structures called biofilms. Studies suggest that almost all proximal colon cancers — nearly 90% — are associated with such biofilms, Sears said.

The proximal colon may be particularly vulnerable to these changes because it endures the greatest spillover of antibiotic drugs from the small intestine, Sears said. Then, as the drugs move through the colon, their molecules steadily break down. That said, these potential mechanisms still need to be studied further, but for now, the new study strengthens the case that some link exists between antibiotics and colon cancer, she said.